Conventionally, in an ordinary two-stroke gasoline engine which has been used in a portable power working machine such as a lawn mower and a chain saw, a spark plug is placed at a head portion of a cylinder, an intake port, a scavenging port and an exhaust port which are opened and closed by a piston are formed at a barrel part of the cylinder, and one cycle of the engine is completed with two strokes of the piston without inclusion of independent strokes for only intake and exhaust.
In more detail, by the ascending stroke of the piston, an air-fuel mixture is taken into a crank-chamber under the piston from the intake port, the air-fuel mixture is pre-compressed by a descending stroke of the piston, the aforesaid pre-compressed air-fuel mixture is blown out into a combustion operation chamber above the piston from a scavenge port, and thereby, combustion waste gas is discharged to the exhaust port, in other words, the combustion waste gas is scavenged by using the gas flow of the air-fuel mixture.
Therefore, unburnt air-fuel mixture is easily included in the combustion waste gas (exhaust gas), the fresh gas (unburnt air-fuel mixture) which is directly discharged into the atmosphere without being used for combustion, a so-called blow-by amount is large, not only fuel efficiency is low as compared with a four-stroke engine, but also large amounts of HC (unburnt component of a fuel), CO (incomplete combustion component of a fuel) and the like which are harmful components are included in the exhaust gas. Thus, even if the engine is compact, there can be problems of concerns about environmental contamination, and how to respond to the exhaust gas control which is going to be increasingly strict henceforth and the demand for improvement of fuel efficiency.
In view of such problems, there have been conventionally proposed various techniques of restraining the aforesaid blow-by by improving the shape, structure and the like of scavenging passages as seen in, for example, JP Patent Publication (Kokai) No. 09-088617 A (1997), JP Patent Publication (Kokai) No. 2006-348785 A, JP Patent No. 83041 (Patent Showa 4 (1929)), JP Patent Publication (Kokai) No. 2009-299605 A and the like.